Spaced direction finding of nighttime whistlers at low and equatorial latitudes and their propagation mechanism

Abstract

The propagation mechanism of low‐latitude and equatorial latitude whistlers is investigated on the basis of spaced direction finding measurements in South China. Observations were made continuously in the local time (LT) interval from 0000h to 0400h during the period of January 5–11 1988 at the three stations (Zhanjiang (geomagnetic latitude 10.1°), Guilin (14.1°), and Wuchang (19.4°)) and two horizontal magnetic components and one vertical electric field component were simultaneously recorded over a wide frequency range to enable comprehensive direction finding. Two major occurrence peaks on January 5 and January 6 and two minor ones on January 9 and January 11 have been analyzed and the following experimental results have emerged. (1) The whistler occurrence at very low latitudes is generally very small compared with that at low latitude (geomagnetic latitude ≳20°), but once it occurs, the occurrence rate becomes comparable to that at low latitudes. (2) The whistler dispersion is single‐valued at any particular local time. (3) The ionospheric exit region of whistlers is very much restricted to the geomagnetic latitude range of 10°–14°, and there are no observed whistlers whose path latitude is between 14° and 20°. (4) The extent of their ionospheric exit region is very stable on different days, and the radius of the distribution is less than 40–50 km. (5) Surprisingly high occurrence of echo train whistlers is observed. (6) The propagation of whistlers in the Earth‐ionosphere waveguide after ionospheric transmission is more likely toward higher latitudes than toward the equator and the subionospheric propagation seems to exhibit a horizontal beaming around the magnetic meridian plane. We attempt to interpret these characteristics of the observations in terms of either nonducted or field‐aligned propagation, but discussion indicates that especially findings 4 and 5 are strongly indicative of field‐aligned propagation for very low latitude whistlers localized in geomagnetic latitudes of 10°–14°.